Petite Cactus 3D Thank You Cards Holographic Original Recording System And Preparation

Petite Cactus 3D Thank You Cards Holographic original recording system and preparation

 

The experimental system used for original holographic recording generally includes shock-absorbing holographic table, optical path steering device, beam splitter, beam expander, collimator, imaging lens, Fourier transform lens, pinhole filter, dry plate frame, reset frame, shutter, automatic exposure and development fixer timer, multi-degree of freedom trimmer, variable light bar and other components.

 

Holographic table

Hologram is a record of interference fringes of reference wave and object light wave. If the difference of optical path between the two light waves changes during exposure, the modulation of interference fringes will be affected. A change of less than one tenth of the optical path difference is usually required. Therefore, in the holographic original production process, special attention should be paid to the stability of the holographic workbench.

 

The factors affecting the stability include vibration, air flow and thermal change. The main impact of vibration comes from the vibration of the ground, if the mechanism of the recording system component is loose, it will amplify the vibration, so it is necessary to take damping measures for the holographic workbench. The special holographic air floating table is the best damping table. Simple shock absorption methods can be sand boxes, microporous plastics, air cushions (with the inner tube of the wheel of a car or an aircraft) and cast iron or granite weighing 100 ~ 200kg, and an isolation cover should be installed. If there is no isolation cover, the room should not be ventilated when recording the hologram, the staff should not speak loudly and stay away from the workbench.

 

The method to check the stability of the table is to use the recording system components, arrange a long-arm Michelson interferometer system, adjust to only 3 to 5 interference fringes, and then watch the fringes, if there is vibration outside, the interference fringe does not change, it can be considered to achieve vibration suppression. Then put one or two mirrors with a cushion, if after a period of time the fringe does not change, it is considered that the shock is good. In order to further determine whether the hologram is seismoidal, the position and direction of the mirror can be changed several times. If the interference fringe does not change, it can be determined that the shock elimination is achieved. When pulsed laser is used as the recording light source, because the exposure time is very short, there is no need to set up shock absorption and shock-proof measures.

 

Optical element

Optical components such as lenses and mirrors made by holograms work on the same principle as optical components for ordinary purposes.

 

1. In order to improve the transmittance of light and prevent the noise of hologram reproduction caused by useless reflection, the lens surface should be coated with anti-reflection film. In addition, when selecting a lens, it is necessary to choose a lens without defects and dirt, because they will cause noise when the hologram is recorded and reproduced.

 

2. When the light is incident on the glass base of the ordinary mirror, it must first be refracted and then reflected, and the loss of reflected light is very large. At the same time, both sides of the glass substrate will introduce stray light due to multiple reflections. Therefore, holographic plate making requires a high reflectivity mirror with high surface flatness and multi-layer reflective film.

 

3. The beam splitter is an important component of the recording system, and its role is to divide the laser beam into two or more beams in order to serve as a reference wave and the illumination wave of the object. The beam splitter is usually plated with an interference film on a long glass plate. There are two kinds of interference film: multilayer dielectric film and metal film. The spectral ratio can be continuously changed or segmented. The beam splitter made of polarization element can continuously adjust the beam splitter ratio, and in some cases it is necessary to divide the beam into three or more beams. One method is to use several beam splitters, and the other method is to use multistage diffraction of holographic gratings. The phase holographic grating can also be used as a beam splitter with continuous change of beam splitter ratio, which can be changed continuously as long as the incidence Angle of the laser is changed.

 

4. Because the divergence Angle of the laser beam is very small, it is necessary to use an expanding mirror to increase the divergence Angle of the beam. Usually used 20 times, 40 times the micro objective lens, or a single positive or negative lens with a very short focal length. Sometimes, in order to reduce optical elements, convex or concave spherical mirrors are used to expand the beam and change the direction of the beam at the same time.

 

If a positive lens with a focal length of f 'is used for beam expansion, and the spot diameter of the laser beam is 2, the beam divergence Angle after beam expansion is. Since the intensity distribution of the laser beam is Gaussian, the intensity distribution is still Gaussian after the beam expansion. In order to make the distribution of light intensity uniform, generally add a light bar in the light path. Thus, by using only a small part of the beam, a lot of energy is lost.

 

5. The role of the pinhole filter is to limit the size of the beam and eliminate the high noise generated by the beam expanding mirror and the optical elements that the beam passes through before expanding the beam. The pinhole filter is generally a molybdenum sheet with a thickness of 25, which is made into a pinhole of 5 to 30μm by laser drilling. The choice of the size of the pinhole is related to the size of the speckle on the focal surface of the beam expanding mirror and the size of the record. Without considering the resolution of the image, the pinhole can be selected by referring to the above formula. Because the beam expander has certain aberrations, the actual spot is larger than the value calculated above, and the pinhole size can be approximately calculated. The pinhole should be placed on the bright spot on the rear focal surface of the beam expanding mirror when in use. Usually the pinhole and the beam extender are mounted on a bracket, and the position of the pinhole can be adjusted in three mutually perpendicular directions. First, the pinhole is placed near the focus of the beam expanding mirror, and a light screen is placed behind the pinhole. First slightly move the vertical (or horizontal) direction of the adjustment handwheel, with the horizontal (or vertical) direction of the handwheel to scan; If there is no light spot on the screen, continue to repeat, until there is a light spot, move the position of the pinhole along the optical axis or vertical direction to gradually increase the brightness of the light spot, and fine-adjust the horizontal or vertical direction of the handwheel, so that the concentric light and dark diffraction ring is observed on the light screen; Finally, fine-tune along the direction of the optical axis, so that the central bright spot radius continues to expand, the brightness gradually increases until the brightest.

 

6. Scatterers Ground glass, plaster, white paper, etc., can be used as scatterers. A scatterer is used to obtain a uniform scattered light field. First, the illuminance of the scatterer is required to be uniform so that the scattered light is uniform. Good scatterers in the direction of the luminous flux and the normal direction of the luminous flux difference is very small, meet the relationship, so that in the range of =±20° and the difference is less than 6%.

Holographic original recording system preparation

1. Preparation of the object to be photographed Holography commonly used photographic objects are mostly small, shiny and highly reflective objects. For objects such as ornaments made of glass, a thin layer of grease is applied to the surface before shooting, so that it has diffuse scattering characteristics. If a helium-neon laser with an output power of 10mW is used, the size of the illuminated object can be up to 20cm×30cm. If a mode-selected argon-ion laser is used, the object can be up to 30×50cm. If a giant pulse ruby laser is used, a human image can be taken.

 

2. Choose a dry plate sold on the market labeled with the laser for which it is suitable. According to the customer's requirements for the type of holographic trademark and the layout color design requirements, the dry plate that is suitable for the wavelength and sensitivity of the recording light source should be selected. If the price is right, choose a high resolution dry version as much as possible.

 

3. Laser preparation and adjustment Although the laser beam has a high degree of coherence, it is still necessary to check and adjust the laser before holographic plate making, so that they can correctly match to achieve the ideal working state. There are five main items for laser inspection:

 

① Whether there is stray light in the case of a single transverse mode;

 

② Whether the oscillation output power reaches the specified index;

 

(3) Whether it is a single longitudinal mode, if it is a multi-longitudinal mode output, it is necessary to check what kind of multi-longitudinal mode;

 

④ Whether there is noise mixed in the oscillation output;

 

⑤ Whether it is a stable continuous oscillation.

 

Of the above five checks, the first three are particularly important. Because it has a great impact on the quality of the hologram reproduction image, it should be checked emphatically. After inspection, if it is found that it does not meet the requirements, it should be adjusted.

 

(1) The selection and adjustment of transverse mode holography requires a light source with good spatial coherence, and the spatial coherence of the laser beam is determined by its transverse mode. The phase difference of laser beam wavefront in the same transverse mode is fixed. The laser used for holographic recording is always selected as a single transverse mode. If the laser beam is not a single transverse mode but a multi-transverse mode laser beam, then uniform interference fringes cannot be formed on the hologram. When reproduced, a horizontal pattern will appear, and markings will appear on the image.

 

 

There are three main reasons for multiple transverse modes:

① The design of optical resonator is unreasonable;

 

(2) Attaching dust to mirrors and Brewster Windows;

③ The parallelism of the resonator is not good.

The adjustment method is as follows: First, turn on the power supply so that the laser emitted by the laser shines on a piece of white paper, and observe whether it is a single transverse mode. If it is not a single transverse mode, analyze the reason. If it belongs to the first case, the external cavity He-ne laser can be replaced with a mirror whose curvature radius is suitable for the discharge tube. If it cannot be changed, the parallelism of the mirror can be adjusted slightly by observing the laser output pattern until the single transverse mode output is obtained, and then fixed, or between the discharge tube and the mirror, put a pinhole with a diameter of about 2 to 3mm, so that the laser beam is shot through the pinhole to the mirror to adjust. For ruby lasers, 2 ~ 4mm pinholes can be inserted between the ruby rod and the mirror, and the position of the holes can be adjusted and fixed by observing the laser oscillation pattern. Pinhole material can be thin metal plate. The size of the pinhole is determined by the diameter of the ruby rod and the size of the oscillation, so the optimal value will vary. Various types of pinholes of different diameters should be prepared for use.

 

If there is a complex pattern in the spot, it is mostly because of dust on the outside of the laser output mirror. If this is the case, you can use a hair dryer to blow air on the mirror surface of the mirror and the window of the Bluster window, or use compressed nitrogen to blow away the dust. If the dust is attached to other dirt, you can use a cotton ball or mirror paper with acetone or ethanol, ether mixture (ethanol, ether ratio of 6:4 or 5:5) to vigorously shake off the excess liquid, gently wipe the mirror, pay attention to do not scratch the mirror, wipe the mirror paper or cotton ball after a replacement.

 

If it is the third case, as in the first case, by observing the laser oscillation pattern, the mirror of the resonator is slightly adjusted so that the two mirrors are parallel. In the single transverse mode, the laser output power is maximum. If the two mirrors are still not parallel, their parallelism can be adjusted in the following way.

 

① Make a small hole with a diameter of about 1 ~ 2mm with the appropriate thickness of paper, and draw several concentric small holes with a spacing of about 1mm on it.

 

② Arrange the discharge tube, ND color filter, mirror, flashlight and pinhole according to the situation shown in Figure 5-9, switch on the power, and the discharge tube is in the discharge state. Put the pinhole in a position where the center of the discharge tube can be fully seen, and observe whether the concentric circle image drawn on the pinhole cardboard reflected on the mirror is clear. If you can not see clearly, the flashlight can be used to illuminate the pinhole cardboard, and then adjust the mirror, so that the image of the concentric circle and the discharge tube are concentric (that is, coaxial), then the discharge tube will be vertical with the mirror. By adjusting the other mirror in the same way, the parallelism of the resonator can be adjusted well.

 

Note: Since the above method is to adjust the parallelism of the resonator by observing the mirror with the eye, the moment the mirror is adjusted to be parallel, a strong laser output will be produced to stab the eye. So, as you approach the desired parallelism, move your eyes away and adjust by turning the reflector's adjusting screw in the desired direction of mirror movement. For external cavity lasers, an ND filter can be inserted between the Brewster window and the mirror or protective glasses can be worn to adjust. If it is a cavity type He-ne laser, the discharge tube is deformed due to thermal expansion, so that the transverse mode of the laser beam changes and reduces the output power, then the installation state of the discharge tube can be slightly adjusted to reach its normal output power.

 

Check whether the resonator has light leakage other than the required output laser, then use a thick black cloth to block the light. If the light leakage is bluish white, this is the light generated by the discharge tube discharge, and a small hole light bar can also be set at the laser outlet to eliminate it.

 

(2) Longitudinal mode selection and adjustment for the use of small He-Ne lasers as recording light, it is not necessary to conduct special longitudinal mode selection work. If large helium-neon lasers, argon-ion lasers and ruby lasers are used as recording light sources, especially when recording large scene objects requires a long coherent length, the longitudinal mode must be selected and adjusted, and a single longitudinal mode laser beam must be used.

 

(3) Reduce the noise if: ① the discharge current of the laser is inappropriate; (2) There is dust or air flow between the mirror and Brewster window; ③ In multi-mode oscillation, beat frequency is generated between modes. In the output power of the laser, the noise that changes with time will be mixed, and the coherence of the laser beam will become worse.

 

The noise reduction method is as follows: if the noise is generated from the first reason, for the He-ne laser whose discharge current can be changed, the laser output can be observed with the oscilloscope received by the optical galvanometer, and the discharge current can be adjusted until the output power of the laser reaches the maximum and the noise is reduced to the minimum. For the helium-neon laser with fixed discharge current, the primary end of the excitation current can be equipped with a variable resistance regulator to adjust the discharge current. If it is the second reason (only external cavity or semi-external cavity laser, there is this noise phenomenon), it can be solved by blocking and external air convection.

 

If it belongs to the third reason, it can be solved by the single longitudinal mode selection method.

 

In addition to the above three main reasons, if the excitation power supply is unstable, the excitation power supply voltage is inappropriate, the discharge tube and the mirror are not fixed, and the service life of the laser is up, it is also the cause of noise. Therefore, we should carefully analyze the causes of this kind of noise and adopt different methods to solve it.

 

4. Optical components and other components

According to the type of recorded holographic original and the design requirements of the recording optical path, a configuration diagram is drawn first. Then select the number of beam splitters, pinhole filters, convex lenses, and full mirrors, prepare a dry plate frame, a second or 1/250s shutter, and a laser power meter that can measure microwatts to ten microwatts. In addition, prepare diffusing plates such as wool glass plates.

 

5. Configure a recording optical path

(1) When the beam direction is parallel to the working table, the beam direction should first be parallel to the working platform. The height of the laser beam should be moderate. If the height of the laser output is not appropriate, apply the lift (or drop) light mirror adjustment to the appropriate height, using the fine-tuning mechanism of the reflector or the lift mirror and a movable bracket with a fixed height marker to adjust the laser beam parallel to the work surface.

(2) After the selection of beam splitting ratio of beam splitting is adjusted, the beam splitter should be installed. The selection of the beam splitting ratio of the beam splitter should take into account the size of the recording object and the surface scattering ability. The ratio of beam intensity at the surface of the recording medium is kept in the range of 1-10.

(3) The general formula for the modulation regime of the interference fringe of the polarization direction of the laser beam is, where is the ratio of the reference beam to the object beam, is the Angle between the vibration direction of the reference beam and the object beam, and τ(t) is the time coherence of the reference beam. The above formula shows that polarized light that is perpendicular to each other cannot produce bright and dark stripes. Because of the Brewster window, the laser output is polarized light, and the polarization direction should be perpendicular to the incident plane of the beam. Although the output laser beam of the cavity laser is not polarized light, there is a characteristic that the light intensity in a certain vibration direction is the largest, so this direction should be perpendicular to the incident plane.

(4) The laser beam should be adjusted by the center of the beam expanding mirror and the collimator, when the beam expanding mirror is installed, the beam expanding mirror should be adjusted up, down or left and right (without pinholes) so that the center of the emitted light spot coincides with the laser beam; When using a collimator, the position of the center of the lens should coincide with the center of the laser beam before the laser beam is expanded. The method is to see if a series of points of light reflected from the two surfaces of the lens are in the same line.

(5) The stable installation of various components of the mechanical structure of the various optical elements of the bracket mostly has 2 to 4 degrees of freedom adjustment, in use to check whether the parts of the mechanism is loose, the fixing screws to tighten, the adjustment of the part to be installed in the middle position is good, the bracket is generally used magnetic table, record it and the table firmly.